Sucker rod



May 9, 1933.

WITNESSES (i hi EIXTCDN SUGKER ROD Filed March 13, 1928 TNVENTOR $1. 2m, 5am

Patented ay 9, llQd teeter GEORGE at. narrow, or rrr'rsnunen,PENlISYLVANIA, nssrenon are THE NATION surrLY comrm, or 'roLEDo, one, acoaroaarron or onto non My invention relates to improvements in jointsfor rods, more especially to sucker rods for pumping oil wells. In theoil well industry, sucker rods are a thoroughly recognized part of thestandard equipment, consisting of a string of about thirty foot lengthscoupled together end to end and extending from a source of reciprocatingmotion above the ground down to a pump whose suction connection dipsinto the oil pool.

In shallow Wells the sucker rods give very little trouble. But as thewells are driven deeper, the sucker rods are subject to an increasingamount of stress resulting infallure. Sucker rods are joined by screwingthe threaded end of one rod into a threaded socket end of another rod.The majority of failures are caused by the breaking ofi of the stud endof the rod at or near the end of its thread.

In accordance with the present invention the failures referred to arelargely prevented. by remedying the cause from which.- they originateThe invention will be more clearly illus trated in connection with theaccompanying drawing in which like reference characters designate likeparts and in which F g. 1 shows an elevation, partly in section, througha sucker rod coupling, embodying the prmclples of my inventon; Fig. 2 anelevation.

artly in section, t rough a sucker rod emdying an improved form ofjoints Fig. 3 an end view of one of the half coupllngs of Fim Q; andFig. 4 a diagram of the stress distribution at the end of. the thread ofthe coupling which is at the point where failures most frequently occur.

Referring to Fig. 1 of the drawing, sucker rodsare made from a rolled ordrawn round 1. This round is cut into lengths and each length is upseton each end. With present practice this upsetting forms a socket end2and a stud end 3 on the respective ends of the rod 1. Wrench andelevator holds are formed in the upsetting but are not described indetail as they are not relevant to this invention. The socket end 2 hasa thread 4 and the stud .end 3 a thread 5 adapted to dealt with occur atthe end of the thread and are due to a phenomenon known in the scienceof mechanics, as stress concentration. en a cylindrical piece of elasticmaterial which is grooved circumferentially, or approximatelycircumferentially, is subjected to direct tension or to bending, theactual stress at the bottom ofthe groove is very much greater than isindicated by the usual stress formulae. This fact has been definitelyproven by means of the photo-elastic analysis. The intensity of thestress concentration is a function, among other details, of thesharpness of the bottom of the groove, and of the depth of the groove.The seriousness of this stress concentration is a function of thephysical characteristics of the material and also of the way in whichthe load is applied.

1 have discovered that under the conditions existing in the end thread,the stress 9 at the root of the thread, as diagrammatically il-vlustrated in Fig. 4, is over twice as great as the average stress foundby applying the usual formulae. If the metal is extremely ductile, andif the local 'stress exceeds the elastic limit, the stressed metalundergoes plastic flow and the stress is relieved, dropping down to theelastic limit. If the assembly is permanent, and if the plastic flowreferred to occurred under the influence of assembly forces which are inexcess of any working loads imposed by service, the structure may beentirely safe.

But in the case of a sucker rod installation, the assembly is notpermanent. During the pumping operation the well becomes fouled withsand and with deposits from the oil, and itis necessary to withdraw thestring of sucker rods to clean the well. In the operation of withdrawingthe string itis necessary to uncouple many of the rod-to-rod joints.When the axial load is removed by the act of unscrewing, the stress 9 atthe critical point drops 0E, and on reassembly again rises. Since thescrewing up and unscrewmg arehand operations, wide force variations willoccur and further plastic flow of the nature referredto will inevitablyoccur. Furthermore, the sucker rod comes out from a well thickly coveredwith'oil which carries a furtherconcentration of stress at the region ofthe critical section which centers in the axial plane passing throughthe foreign material that has been caught on the mating surface 10. Thisproduces further plastic flow in that part of the critical sectioncentering in the axial plane noted. On the next overhaul of this suckerrod joint, foreign material will lie in some other axial plane andplastic flow will attack a different part of the critical section. It isthus evident that during the life ofa sucker rod it is subjected to anerratie cycle of localplastic flow.

It is the purpose of myinvention to relieve this destructive plasticflow cycle, since repeated plastic flow of the nature described, leadsinevitably to ultimate failure. In Fig. 1 I have shown a method ofimproving existing sucker rods in which the seeds of failure have notalready been planted by the cy-' cle already described. In practicingthis method, the threads 5 are cut away clear to the root 11 or even toa somewhat smaller diameter, for an axial distance 12, which is, in thecase shown, about half of the distance 14 from the shoulder 10 over theend of the thread 5. There remains a length of,active thread 13 which iscapable of carrying over twice as muchaxial load as the body 1 of thesucker rod. It is utterly essential that the machining operation leavethe surface 30 in a smooth condition, since a sharp tool mark at theroot 16 of the fillet 17 would again permit stress concentration to setup the fatal plastic flow. Photo-elastic analysis has further proventhat under the conditions of structure shown and described in Fig. 1there is only a very slight stress concentration at the end 18 of thestud thread 5. It is therefore evident that in a sucker rod constructedunder the principles of my invention there is a very material reductionin the stresses to which the critical section 6 is subjected by theexigencies of the service cycle involved in the operation of suckerrods. The socket end 2 is preferably counterbored at 31 to reduce thenumber of turns required in the assembly and dismembering of the rodjoint.

While the structure shown in Fig. 1 is'of advantage in that it permitsthe improvement of existing sucker rods, as well as the making of newrods which are superior to those now It is unreasonable to expect inuse, there remains a small but unnecessary stress concentration at 16.Fig. 2 shows a construction which eliminates this stress concentration,while permitting the use of, existing forging dies for upsetting theends of the rod 1. In this construction a socket end 2 is formed on eachend of the rod 1. The machining of the ends 2 differs in severalparticulars from that involved in rods of the type indicated in,Fig. 1.A flat seat 19 is machined at a definite distance 20 from the matingsurface 10. The diameter 21 0f the recess forming this seat is equal tothe diam-' eter of the tap drill for the thread 4. A fillet 22 joins thebore 21 of the tap drill and the surface of the seat 19. The seat 19 isused in only one of the ends 2, but it is preferably machined in' bothends, because it is formed in the same operation as the counterbore andit therefore cheaper to include it in both en s.

The ends 2 are secured together by an inde V pendent stud 23. This studhas a reduced diameter 11 extending over its'mid-length with a characterof surface described in con nection with the length 12 of Fig. 1. Italso has an extension 24 on one end. The diameter 25 of this extension24 is slightly less than the diameter of the tap drill for the thread 4.This extension 24 serves the double purpose of a pilot for aligning thestud 23 with the mating surface 10, and also of assisting in locking thestud 23securely in place in the end 2.- It is clear that if the stud 23were screwed hard home in the thread 4, and with its end 27 abutting'ard on the forged surface 26 (Fig. 2) of the r cess, misalignment wouldbe sure to occur. This misalignment would produce improper mating of thesurfaces 10,.

with resulting stress imposed on the stud 23.

But with the square and flat seat 19 the stud,

.The upper socket member 2 of Fig. 2 is shown expanded, it beingslightlylarger than the lower member 2. When a balanced temperature hasbeen secured in the expanded socket 2, the stud 23' is run home quickand hard with a stud driver. and with its end 27 bottoming hard on theseat 19. The assembled members 2 and 23 are then allowed to cool. It ispossible to use a temperature of from 300 to 400 degrees F. for thisheating-operas tion with the structure which I prefer.

Ordinarily in locking a small male screw I into a female thread. byshrinking, it is necessary to perform very precise machining operationstomsure that the female thread g52is'co'oled downthese tension andcompresthreads as machined) are such that at any -temperature within thepractical range, the

stud goes home readily. The final screwing home produces the'relativepositions of the stud threads 5 and the socket threads shown (in anexaggerated degre')" -in Fig. 2. The stud thread 28' first makes hardcontact with the socket thread 29. This contact travels back awayfromthe stud end 27 by a distance dependent upon the forces involved and theelastic characteristics of the stressed metal. The stud extension 24 isput into axial'compression, and the corresponding length of the socket 2is put into tension. When the socket sion forces are increased, and thestud 23 is thus set harder into the end 2 than it is possible to set itwith the stud driver alone. It is evident from an operating standpointthat the stud 23 is ,an integral part of the sucker rod into'which ithas been locked. .Counterbore 40-limits the active area of seat 19 andprotects threads from damage due to overshrinking.

Reference has been made to the'tendency for hard foreign material tobecome trapped between the abutting surfaces 10, and to set upseriesstress concentrations. To reduce this tendency, it is current practice,as shown in Fig. 1 to make a counterbore 32 and thus provide a recess 38to receive such of this foreign material as tends to squeeze inwardlyfrom between the .surfaces 10 as they come into working contact. Thistendency is in current practice further assisted by machining thechamfer 35which increases the capacity of the recess 38 and at the sametime reduces the dirt entrapping area of the surfaces 101 In actualpractice however a condition arises which is worse than that which thechamfer 35 is intended to correct. The exposed sharp edge becomesba-dlyscarred, and the degree of this scarring is often so great that theresulting burrs prevent the proper alignment of the surfaces 10.To-correct this defect, I partially replace the inner chamfer 35 withthe outer chamfer 37. This presents a surface far less susceptible todamage and preserves the principle of reduced mating surface 10. Therecess 38 is also of muchgreater capacity. than has been available priorto my invention.

. The amount of dirt actually hard trapped between the surfaces 10 isstill. further re-' duced by interrupting the circular continuity ofthese surfaces by grooves 39 which are formed in the end 2 during theupsetting operation.- These grooves 39 are shaped to flare outwardly andwill. become completely filled with a mixture of thick oil and grit, butthe pressure setup in screwing the rod ends together will always besuficient to introduce 'new material into the grooves by forcing outenough eld material for its accommodation.

It is evident that with the practicing of my invention sucker rods ofcarbon-steel be come practical for wells deeper than those Where carbonsteel rods are now entirely satisfactory. When thedepth of well reachesa point where rods as shown in Fig. l are no longer satisfactory,the'construction of Fig.

2 may be employed since the stud 23 may be made of the highest grade ofalloy steel without'a serious increase in the cost of a string of rods.When the depth of well becomes so great that epidemic failure occurs inthe body-of the rod 1, the entire rod may be made of alloy steel. Whenthis is done I prefer to employ aIi air hardening or normalizing steelin order avoid the stressconcentrations necessarily accompanying thequenching of a piece of the shape of a sucker rod with its radicalchanges of cross section. Also the heating operation involved in locklngthe stud 23 into the socket 2 becomes entirely free from the danger ofdeteriorating the material of the end 2, when the rod is'in a norwhich Iprefer. i

I claim herein as my invention:

1. In a sucker rod a removable coupling comprising a pair of socketends. provided with screw thread portions, and a stud having threadedportions interacting with the threads of said sockets, said studhaving'a constricted body portion intermediate its threaded portions andbeing further provided with a constricted portion at one end thereof toabut against the bottom of one of said socketsf g r g 2. In a ucker rodaremovable coupling comprising a rod having a socket end provided with ascrew thread portion and having a plurality of grooves extendingradially through the wall of the socket, a second rod having a threaded.stud end ada ted to interact with the thread of said soc et, said secondrod having anabutment for engagement with the end of said socket andforming clearance spaces with the grooves of said first named rod. i

3. A sucker-rod comprising a plurality of rod sections having socketends provided with threaded portions and a stud member having threadedportions adapted to interact with the threads of said socket ends tojoin said sections with their ends in abutment and having its bodyportion radially spaced from the walls of the socket members at theirpoint of abutment, said stud engaging said malized condition with theclass of steel threaded portions at portions spaced from said abuttingends.

4. In a well drilling coupling member of the type comprising acylindrical body having an integral extension at one end, an endfacingshoulder at the inner end of said extension, and a screw thread on saidexten sion; means to distribute the breaking strain previously localizedat the last turn of said thread, along an extended portion of saidextension, said means comprising a zone of substantial width betweensaid shoulder and said thread, said'zone being of a diameter no greaterthan that of the valley of said last turn of the thread.

In testimony whereof, I have hereunto set my hand.

GEORGE M. EATON.

